Tom,I would absolutely create these 9 servers in a single cluster, but that doesn't necessarily mean that each database has an instance on each node. You can (and I would) build a 9-node cluster, but have database instances running on nodes 1-2, a different database on node 3-7, and a third database running on nodes 8-9. This would/should allow you to bring up an instance for database #1 on another node if node 2 were to fail. If they are separate clusters, that would not be possible.
This is along the lines of what Oracle's Grid architecture intends to promote. I would not advocate running all instances on all nodes when you have enough nodes to avoid such a case. Oracle's resource management is only within a single instance (at least in currently-available releases), so two instances on the same node can and will have potential to suck up all resources on the node without regard to the other instance(s) on that node.
The other questions that Tom M. asked are good ones and should be part of your consideration as well. In general, even before I knew the answers you gave, I'd advocate an architecture similar to what I've described above.
Dan Terrian, Thomas J Mr CTR DLA J6DIB wrote:
Are these three production databases? YES Any development or staging to worry about? YES, they are on other machines on one cluster. Does any individual database have a higher profile than the others? Not sure what you are asking. Does it need to be alone for political/business reasons? Good question, right now each program manager wants his own cluster. We are looking to see if it may be better for the organization to just set up one production cluster. Do any of these databases hold a warehouse? Are any a reporting only database? NO, OLTP.